how does Twin-T oscillator work

 Twin-T oscillator is one form of RC oscillators. There are many classification of oscillators. Oscillators designed using resistor and capacitor are called RC oscillators. Other types of oscillators which uses other types of components like inductor and capacitors are called LC oscillators and oscillators which uses crystals are called crystal oscillators. Furthermore crystal oscillators can be Oven-controlled crystal oscillators (OCXOs) or temperature-compensated crystal oscillators (TCXOs) and so on. Similarly there are many forms of RC oscillators such as Wein Bridge oscillators, RC phase shift oscillators etc and Twin-T oscillator is one kind of RC oscillator. Here Twin-T oscillator using operational amplifier is explained(alternative would be twin t oscillator circuit using transistor), its uses is explained, its frequency range is explained, twin-T oscillator advantage and disadvantage are explained.

The Twin-T oscillator circuit diagram designed with operational amplifier is shown below.

Twin-T oscillator circuit diagram

Like any other oscillator the twin-t oscillator is build using the feedback network and an amplifier. The twin-t circuit forms the feedback network and the op-amp is used as the amplifier. In the twin-t oscillator circuit drawing above, the positive feedback is provided by the potential divider of R1 and R2(which is itself a potentiometer to vary resistance). The negative feedback is provided through the twin-t circuit.

The twin-t circuit diagram is shown below.

twin-t circuit diagram

The twin-t circuit consists of two equal resistors R and two equal capacitors C. Between the equal resistors R is placed a capacitor whose value is twice the value of C, that is 2C. Similarly between the equal capacitors C is placed a resistor whose value is half the resistor R, that is R/2. This twin-t circuit is also known as notch filter or passive band-stop filter(see How to design Twin T Active Notch Filter?). Basically this twin-t filter is made up of high pass filter and low pass filter. It is also known as lead-lag circuit.

The resonant frequency of this twin-t circuit gives is the frequency of oscillation of the entire Twin-T oscillator which is given by the following equation.

\[f_{r} = \frac{1}{2 \pi R C}\] 

Twin T Oscillator Calculator

 The following is online Twin-T oscillator calculator that calculates the resistor value for given resonant frequency and capacitor.


Theoretical Results:
It's phase angle varies between +90 to -90 degree against frequency. Also at the resonant frequency fr, its phase angle is 0 degree and does not introduce any phase shift. The gain is unity(1) at low and high frequencies but at resonant frequency fr the gain is 0.

How does the twin-t oscillator work is explained now. When power is applied to the oscillator circuit, and the resistor R2 value is low enough, a high positive feedback is created which assists the circuit to build oscillation. With proper increase in R2 the positive feedback is decreased and with proper adjustment of R2 the oscillation can be controlled to obtain a sustained and stabilized oscillation. 

For twin-T oscillator oscillation other than at resonant frequency fr is not possible because at other frequencies other than fr, there will be negative feedback and hence the oscillation will not happen(oscillators need positive feedback). To help in the setting of value of R we can use a potentiometer in place of R/2 resistor indicated in the circuit diagram. Also the value of the resistor R2 should be large to obtain oscillation. Usually value of R2 is in the range of 10R1 to 1000R1.

So the disadvantage of using twin-T oscillator is that it oscillates only at a specific frequency. 

See other oscillator tutorials below.

- Design and Simulation of LM358 based Voltage Controlled Oscillator(VCO) 

- Practical Colpitts Oscillator on Breadboard

- Colpitts Oscillator Design and Experiment on Breadboard 

 Colpitts Oscillator Simulation in Proteus

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